1. Investigación

Oestrogens treatment is often used to induce oestrus behaviour in anoestrous mares to aid in the collection of stallion semen and as recipient mares to receive embryos when combined with progesterone. However, there are no studies to describe the effect of dose and individual mare on the intensity and duration of the response, in both anoestrous and cyclic mares. In Experiment 1, 13 anoestrous mares were treated with one of five doses of oestradiol benzoate (OB) (1, 1.5, 2, 3 and 4 mg) per mare in five consecutive treatment periods (n = 65), to determine the response in terms of endometrial oedema and oestrous behaviour. Experiment 2 and 3 used 3 mg of OB in cyclic mares to confirm or deny the presence of an active corpus luteum (CL). There was a dose rate of OB and individual mare effect (p < 0.05) on the intensity and persistence of endometrial oedema and oestrous behaviour. A total of 2 mg OB was enough to induce endometrial oedema and oestrous behaviour within 48 h in most mares. Mares with an active CL did not show endometrial oedema following treatment of 3 mg OB.

The present experiment aimed at determining whether the timing of the maternal recognition of pregnancy (MRP) was specific to individual mares by determining when luteostasis, a failure to return to oestrus, reliably occurred in individuals following embryo reduction. Singleton (n = 150) and synchronous twin pregnancies (n = 9) were reduced in 10 individuals (5–29 reductions/mare) at pre-determined time points within days 10 (n = 20), 11 (n = 65), 12 (n = 47), 13 (n = 12) or 14 (n = 15) of pregnancy. Prior to embryo reduction, the vesicle diameter was measured in 71% (106/150) of the singleton pregnancies. The interovulatory interval (IOI) was recorded on 78 occasions in seven of the mares in either non-pregnant cycles (n = 37) or those in which luteolysis followed embryo reduction (n = 41). The earliest time post-ovulation at which the embryo reduction resulted in luteostasis in an individual was 252 h (mid-Day 10). Consistency in luteostasis following embryo reduction showed individual variation between mares (272–344 h). Binary logistic regression analysis showed an individual mare effect (p < 0.001) and an effect of the interval post-ovulation at which embryo reduction was undertaken (p < 0.001). However, there was no significant effect of vesicle diameter at the time of embryo reduction (p = 0.099), nor a singleton or twin pregnancy (p = 0.993), on the dependent of luteolysis or luteostasis. The median IOI between individual mares varied significantly (p < 0.05) but was not correlated to the timing of MRP. The timing of MRP varied between the mares but was repeatable in each individual. The factors and mechanisms underlying the individuality in the timing of MRP were not determined and warrant further study.

It is believed that during the spring transition, the developing follicle tends to grow more slowly, persist longer and grow to a larger diameter prior to ovulation than at subsequent oestrus periods. A general suspicion, that the first ovulation of the year is less fertile than subsequent ovulations could be explained by a slower growth rate of the ovulatory follicle during transition with the consequent production of a subfertile oocyte. By detailed serial examination of the same group of Irish Draught mares over three winter/spring periods, no significant difference was found in either growth rate or pre-ovulatory diameter when compared with subsequent ovulations. Mean growth rates over the ten days prior to ovulation were 2.20 mm/day (range 1.18 to 3.64) and 2.19 mm/day (range 1.25 to 3.41) for first and subsequent ovulations respectively. Mean maximum pre-ovulatory diameters were 44.7 mm (range 35 to 59) and 43.5 mm (range 31 to 57.5) for first and subsequent ovulations respectively. The impression gained by practitioners that the first follicle develops more slowly during the transition to the first ovulation of the season may be due to less frequent examinations and consequently a failure to observe and record that follicles may grow and then regress during this period. The largest follicle observed a few days previously is not necessarily the same large follicle found at a later examination.

The effect of three different hormonal protocols to prepare anestrous recipient mares on embryo survival was evaluated. The first group consisted of only progesterone administration (NE) 4 days before embryo transfer, while the recipients from the other two groups received a single administration of 2.5 mg of oestradiol benzoate (SE) 2 days earlier or 8 mg of oestradiol split in increasing doses for 5 consecutive days (LE) ending 3 days before progesterone treatment. The likelihood of recovering an embryo 2 days after transfer was 46.1% (6/13), 62.5% (5/8) and 85.7% (6/7) for recipient mares from the no oestrus, short and long oestrous groups respectively (p = .09). In conclusion, the presence and duration of oestradiol treatment before progesterone administration tended to influence the embryo survival in anestrous recipients 2 days after transfer. The surviving embryos recovered from the three different groups of recipients did not show any difference in size and morphology.

Las prostaglandinas (PGE2 y PGF2α) ejercen un rol esencial en el proceso de la ovulación en los mamíferos. El objetivo de este experimento fue determinar si la inyección intrafolicular con prostaglandina F2 alfa (PGF2α) administrada en el período preovulatorio inducía la ovulación normal y formación del cuerpo lúteo, confirmado más tarde con la presencia de una vesícula embrionaria. Se utilizaron 6 yeguas mestizas en dos ciclos estrales cada una, con un diseño crossover. El tracto reproductivo de las yeguas fue ecografiado cada 24 h desde el día 15 del ciclo (Día 0 = día de la ovulación) hasta el momento de la punción folicular (Hora 0). La punción ecoguiada del folículo preovulatorio (diámetro ≥ 35mm y presencia de moderado edema uterino) se realizó a través del flanco, inyectando 750 μg de PGF2α diluido en 0,5 ml de agua estéril (grupo tratado n=6) o placebo (0,5 ml de agua de inyección) (grupo control n=6). Previo a la punción, las yeguas fueron inseminadas con semen fresco. Se obtuvieron imágenes ultrasonográficas cada 12 h del folículo punzado y se tomaron muestras de sangre cada 48 h de la vena yugular para determinar los niveles plasmáticos de progesterona. Cinco de las seis yeguas (83%) del grupo tratado, tuvieron fallas en la ovulación y sólo una de las yeguas ovuló pero no se obtuvo preñez. De acuerdo con estos resultados, la inyección intrafolicular con PGF2α no permitiría la ovulación normal y, contrariamente a lo esperado, induciría la formación de folículos hemorrágicos anovulatorios. Además, se observó un retraso en la elevación de progesterona en la única yegua que ovuló. / Prostaglandins (PGE2 and PGF2α) play an essential role in the ovulation process in mammals. The objective of this experiment was to determine whether intrafolicular injection with prostaglandin F2 alpha (PGF2α) administered in the pre-ovulation period induced normal ovulation and corpus luteum formation, it was later confirmed with the presence of an embryonic vesicle. Six mixedbreed mares were used in two estrous cycles each, with a crossover design. The ultrasonographic examination of the reproductive tract was performed every 24 hours from day 15 of the cycle (Day 0 = day of ovulation) until the time of follicular puncture (Hour 0). The ultrasound guided puncture of the preovulatory follicle (diameter ≥ 35 mm and presence of moderate uterine edema) was performed through the flank, injecting 750 μg of PGF2α diluted in 0.5 ml of sterile water (treated group n = 6) or placebo (0.5 ml of injection water) (control group n = 6). Prior to puncture, the mares were inseminated with fresh semen. Ultrasound images were obtained every 12 h of the punctured follicle and blood samples were taken every 48 h from the jugular vein to determine plasma progesterone levels. Five of the six mares (83%) of the treated group had ovulation failures and only one of the mares ovulated but no pregnancy was obtained. According to these results, intrafolicular injection with PGF2α at the concentration used would not allow normal ovulation and contrary to expectations would induce the formation of anovulatory hemorrhagic follicles. In addition, a delay in progesterone rise was observed in the only mare that ovulated.